The Rio Grande Rise (RGR) and the Walvis Ridge (WR) make an odd pair. Both are large igneous provinces (LIPs) apparently erupted at the same hot spot during the opening of the South Atlantic Ocean (Morgan, 1971, 1981, 1983). The conjugate LIPs formed because the hot spot erupted at the Mid-Atlantic Ridge (MAR) during the Late Cretaceous, emplacing volcanic edifices on both the South American and African plates (Kumar, 1979; O'Connor & Duncan, 1990). Despite the similar circumstances, the two features have Abstract Rio Grande Rise (RGR) and Walvis Ridge (WR) are South Atlantic large igneous provinces (LIPs), formed on the South American and African plates, respectively, mainly by volcanism from a hot spot erupting at the Mid-Atlantic Ridge (MAR) during the Late Cretaceous. Both display morphologic complexities that imply their tectonic evolution is incompletely understood. We studied bathymetry, gravity, and vertical gravity gradient maps derived from satellite altimetry to trace faults providing indications of seafloor spreading directions and changes. We also examined magnetic anomalies for time constraint and reflection seismic data for structural information. Abyssal hill fabric and magnetic anomaly data indicate that the area between RGR and WR was anomalous between anomalies C34 (83.6 Ma) and C30 (66.4 Ma) owing to reorganization of a right-lateral transform on the MAR. This event began ∼92 Ma as the transform shifted south to form multiple, short-offset right-lateral transforms, with the reorganization extending through anomaly C34 and ending before anomaly C30. Anomalous spacing of magnetic anomalies and discordant fault fabric indicate that a microplate formed with a core of Cretaceous Quiet Zone seafloor. As the MAR jumped eastward, this microplate was captured by the South American plate and now resides mostly in a basin between the main RGR plateau and a related ridge to the east (East Rio Grande Rise). The microplate is ringed by igneous massifs, implying a link with volcanism. The results presented here indicate that these two LIPs had a complex Late Cretaceous history that belies simple hot spot models. Plain Language Summary Rio Grande Rise (RGR) and Walvis Ridge (WR) are large volcanic mountains formed on the South American and African plates, respectively, as the South Atlantic Ocean opened during Cretaceous and Cenozoic time. They are considered to be similar to the classic Hawaiian-Emperor hot spot seamount chain and to represent motion of the two plates over a single mantle hot spot, once located at the Mid-Atlantic Ridge (MAR). Complex morphologies of these two igneous provinces imply that this simple model is an oversimplification. We examine geophysical evidence for Late Cretaceous time, mainly between magnetic anomalies C34 (83.6 Ma) to C30 (66.4 Ma). During this period the MAR reorganized, causing complex tectonics between RGR and WR. A long-offset, transform fault broke into a number of lesser offset transforms with several ridge jumps, beginning at ∼92 Ma and ending before anomaly C30. ...

In the South Atlantic, a reorganization of the Mid‐Atlantic Ridge began before anomaly C34n (83.6 Ma) and ended before anomaly C30n (66.4 Ma), complicating tectonics of Rio Grande Rise and older Walvis Ridge (WR), which formed together at the Mid‐Atlantic Ridge. This reorganization is poorly understood because magnetic anomalies C30n‐C34n are poorly defined near WR. We interpreted these anomalies along western WR to improve knowledge of Rio Grande Rise‐WR tectonic development. Anomaly trends indicate that Valdivia Bank has an E‐W age progression, perpendicular to that predicted by hot spot models. Anomaly spacing and width is irregular and anomalous near WR, implying a series of ridge jumps and possibly a microplate between anomalies C34n and C32n. Eastward ridge jumps transferred microplate lithosphere to the South American plate. This study shows that Late Cretaceous tectonic evolution of the Rio Grande Rise‐WR large igneous provinces was more complex than previously understood.

Valdivia Bank is an oceanic plateau in the south Atlantic formed by hot spot volcanism at the mid-Atlantic Ridge (MAR) (Kumar, 1979;O'Connor & Duncan, 1990). It is part of Walvis Ridge, an aseismic volcanic chain that stretches from the Namibian margin of west Africa nearly to the MAR. Although commonly attributed to volcanism from a deep mantle plume (Courtillot et al., 2003;Hoernle et al., 2015), its morphology is more complex than plume trails such as the Hawaiian-Emperor and Louisville seamount chains (e.g., O'Connor & Jokat, 2015b;O'Connor & Le Roex, 1992;Sager et al., 2021). Off Africa, the Walvis Ridge is a linear ridge (Frio Ridge) trending SW-NE. It turns to N-S by nearly a right angle at Valdivia Bank, and upon resuming its SW-NE

Oceanic plateaus are large basaltic constructions found in all major ocean basins and make up ∼5% of the seafloor (Harris et al., 2014). Most of these marine large igneous provinces (LIPs) are massive crustal emplacements believed to have formed by large scale mantle melting (Coffin & Eldholm, 1994), but the mechanism underlying their formation is not well understood. The mantle plume hypothesis has been widely used to explain plateau formation, attributing melting to the surfacing of a deep mantle thermal plume (Campbell,

Valdivia Bank (VB) is a Late Cretaceous oceanic plateau formed by volcanism from the Tristan-Gough hotspot at the Mid-Atlantic Ridge (MAR). To better understand its origin and evolution, magnetic data were used to generate a magnetic anomaly grid, which was inverted to determine crustal magnetization. The magnetization model reveals quasi-linear polarity zones crossing the plateau and following expected MAR paleo-locations, implying formation by seafloor spreading over ∼4 Myr during the formation of anomalies C34n-C33r. Paleomagnetism and biostratigraphy data from International Ocean Discovery Program Expedition 391 confirm the magnetic interpretation. Anomaly C33r is split into two negative bands, likely by a westward ridge jump. One of these negative anomalies coincides with deep rift valleys, indicating their age and mechanism of formation. These findings imply that VB originated by seafloor spreading-type volcanism during a plate reorganization, not from a vertical stack of lava flows as expected for a large volcano.Plain Language Summary Oceanic plateaus are large, elevated underwater features commonly formed from volcanic material from a hotspot. Valdivia Bank is a Late Cretaceous oceanic plateau in the southeast Atlantic Ocean formed by volcanism from the Tristan-Gough hotspot near the Mid-Atlantic Ridge. The origin and evolution of Valdivia Bank is poorly defined, but new magnetic data suggests the edifice originated through ridge-centered volcanism, with lateral accretion of crust. This is unlike the evolution of a massive volcano, which would be expected to create a vertical stack of lava flows. Magnetic inversion modeling suggests the plateau was formed by seafloor spreading during the formation of anomalies C34n-C33r, with the plateau becoming younger from east to west, rather than north-south as predicted by some hotspot models. Results from International Ocean Discovery Program Expedition 391 paleomagnetism and biostratigraphy confirm the anomaly interpretation.